Turbine blades are the primary elements of wind turbines for converting wind energy into electrical energy. The working principle of the blades resembles that of airplane wings. The blades have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
Airflow over the leading edge of the blade is mainly laminar in an “attached-flow” region. The lift force is generated primarily in this attached-flow region. As the air moves towards the trailing edge of the blade, flow separation occurs and the air flow transitions to a “detached-flow” region where the flow is more turbulent. Flow separation depends on a number of factors, such as incoming air flow characteristics (e.g. Reynolds number, wind speed, in-flow atmospheric turbulence) and characteristics of the blade (e.g. airfoil sections, blade chord and thickness, twist distribution, pitch angle, etc). The detached-flow region also leads to an increase in drag force, mainly due to a pressure difference between the upstream attached-flow region and the downstream detached-flow region.
Hence, in order to increase the energy conversion efficiency during normal operation of the wind turbine, it is desired to increase the lift force while decreasing the drag force. To this purpose, it is advantageous to increase the attached-flow region and to reduce the detached-flow region by having the flow separation nearer the trailing edge of the blade, i.e. in a downstream region of the blade. Also, it is generally desired to have a stable flow separation in order to increase the working stability and decrease noise generation of the blade.
It is know in the art to change the aerodynamic characteristics of wind turbine blades by adding dimples, protrusions, or other structures on the surface of the blade. These structures are often referred to as “vortex generators” and serve to create micro-turbulent regions of airflow over the surface of the blade. This results in flow transition stability in relatively low velocity regions of the blade generally closer to the blade root. At the relatively higher velocity regions of the blade closer to the blade tip, the vortex generators serve to extend the flow separation of the airflow towards the trailing edge of the blade to generate more lift and reduce drag.
Static or fixed vortex generating elements are known. Reference is made, for example, to WO 2007/065434; WO 00/15961; and U.S. Pat. No. 7,604,461. The vortex elements in these references have a defined shape, size, and configuration that does not change and, thus, the versatility of the elements for varying airflow conditions is limited.
Retractable or pivotal vortex generators that are deployed relative to the surface of a blade are also known. Reference is made, for example, to U.S. Pat. No. 4,039,161; U.S. Pat. No. 5,253,828; U.S. Pat. No. 6,105,904; U.S. Pat. No. 6,427,948; and WO 2007/005687.
EP 1 896 323 B1 describes a pivotal vortex generator in the form of a flat member that lies on the flow control surface in a retracted state and pivots at an angle from the surface in an extended state. The vortex generator may be formed from a shape memory alloy that is actuated by a heater. The reference describes that other types of actuators may be used to impart a pivoting action to the vortex generator, including a piezoelectric bimorph actuator.
U.S. Pat. No. 7,293,959 describes a wind turbine blade having lift-regulating means in the form of flexible flaps extending in the longitudinal direction along the trailing edge and leading edge of the suction side of the blade. The flaps are activated at high wind speeds to reduce the lift of the blade. The activating means may be piezoelectric.
Although the vortex generators discussed in the references cited above may be considered “dynamic” in that they are deployed to an active state, the usefulness of the elements in the “at rest” state is minimal.
Accordingly, the industry would benefit from a wind turbine blade having dynamic vortex generating elements that provide enhanced aerodynamic surface characteristics in both an active and non-active state over a wider range of wind and airflow conditions.